A tensionally balanced wire rope, which can also be referred to as a twistless or weakly twisted wire rope or cable, can have a multistrand multilayer structure and can comprise a core cord and a cover layer stranded on the core cord in an opposite stranding direction.
The twistfree or weakly twisted wire rope or cable can be made from circular strands or from flat strands (cf., e.g. German Industrial Standards DIN 3051, Sheet 2, Page 2 and DIN 3070, Page 1).
In the known twist free or weakly twisted wire rope or cable either both the cover layer and the core rope are made from circular strands or the cover layer and the core rope are both made from flat strands. The wire rope or cable made from circular strands has different advantages and disadvantages from those of the wire rope formed from the flat strands.
The wire rope or cable formed from circular strands has the following advantage:
The torsion balance and flexibility may be influenced in a wide range by the number of the strand layers in the wire rope or cable and/or the number of strands per layer and by the choice of the structural form of the individual strands (according to wire number and wire strength whereby all the individual strands in the core wires are exclusively circular wires which may be steel wires of equal nominal bearing strength).
However this wire rope or cable has the following disadvantages:
Since the outer strands of the core cord and the individual cover layer strands contact each other only at points extremely high notch and transverse stresses arise.
Becauae of the positively acting large number of cover layer strands, wire rope or cable of this structure is comparatively more susceptible to mechanical damage of the wire rope or cable surface and/or thrusting of the rope core between the strands of the cover layer even with slight kinking.
The advantages of the wire rope or cable formed from the flat strands are:
Each individual strand layer provides a comparatively large and smooth surface because of the oval shape of its strand.
Because of the large-surface contact of the adjacent strand layers the contacting surface is increased and the loading force per unit area is decreased. The attained optimum adhesion friction in a grooved roller acts in a positive way particularly when the wire rope is used with a drive pulley.
The essential disadvantages of this wire rope or cable are as follows:
Because of the shell-like overlapping strand cover layers and the soft core easily deformable by radial pressure the individual strands of the wire rope or cable diameter are more than proportionally reduced during use.
Because of the oval strand shape the steel wire is subjected to a permanent nonuniform bending and torsion and these effects are further amplified during operation as a result of additional shape changes which cause further flattening of the strands.
Only a very limited cross section oriented torsion balancing between the sum of all the right and all the left strand members is possible.
The complete strength of the core cord (considering all of the strand-layer cross sections including the cover layers) is severely limited by the comparatively large strand gaps and by the core and/or shaping wires which are not load supporting. Thus the breaking strength is definitely limited.
The wire rope or cable because of its structure is comparatively stiff with respect to bending.
A wire rope or cable is known in which two outer strand layers of flat strands and an inner stand layer of circular strands are provided. This wire rope or cable however has the above named disadvantages for the wire rope or cable formed from flat strands.